Battery-life and Output Radio Frequency Spectrum (ORFS) are two important criteria for determining the performance of a mobile terminal, such as a mobile telephone or the like. Both battery-life and ORFS may be adversely affected by a varying Voltage Standing Wave Ratio (VSWR) at the output of a power amplifier in the transmit chain of the mobile terminal. The VSWR may vary due to environmental factors such as the user placing an antenna of the mobile terminal near his or her body. As a result of the varying VSWR, the load impedance seen at the antenna also varies from an ideal load, such as 50 ohms.
For a power amplifier having output power controlled by controlling a supply voltage provided to the power amplifier, when the load impedance is less than the ideal load impedance, the output current of the power amplifier increases, thereby creating an excessive current drain on a battery powering the mobile terminal and decreasing battery-life. When the load impedance is more than the ideal load impedance, the output current of the power amplifier decreases, thereby requiring a greater supply voltage to provide the target output power. At some point, the load impedance may increase such that the supply voltage needed to provide the target output power is greater than the maximum voltage that can possibly be provided by the battery of the mobile terminal. If this occurs during ramp-up for a transmit burst, spectral noise will be generated in the output of the power amplifier when the maximum possible voltage level is achieved and a further increase is clipped. In addition, if the supply voltage is varied to provide amplitude modulation, the hard limit of the battery voltage will truncate the output waveform of the power amplifier and cause severe distortion of the desired modulation pattern.
Accordingly, there is a need for a system and method for detecting and correcting over-voltage or saturation in a collector-controlled power amplifier.